1. Field of the Invention
The present invention relates to a light signal transmitting device for transmitting a light signal between a plurality of circuit boards and between a plurality of devices, and a signal processing device composed of the light signal transmitting device.
2. Description of the Related Art
Conventionally, signals are generally received and transmitted between a plurality of circuit boards and between a plurality of devices via electrical connection. It is, however, hard for the electrical connection to cope with speed-increase of signal transmission due to skew between channels, crosstalk, increase in fan-out and the like.
Japanese Patent Application Laid-Open (JP-A) No. 2-41042 suggests a light data bus in which a light emitting/light receiving device is provided on both surfaces of each circuit board, and the light emitting/light receiving devices on the adjacent circuit boards are spatially connected by light.
Because this technique, however, requires optical-electrical conversion at every communication between the adjacent circuit boards, cost and latency increase, and because free space propagation is used, locating is difficult and the technique has a weakness for crosstalk, dust and the like.
Further, JP-A No. 61-196210 suggests a technique in which circuit boards are optically coupled with each other via an optical path composed of a diffraction grating and a reflecting element arranged on a surface of a plate.
In this technique, however, because light emitted from one point can be connected only with one fixed point, multiple connections cannot be carried out.
In view of such circumstances, JP-A No. 10-123350 suggests a signal processing device in which light scatterers having refractive index different from that of a light transmitting layer are dispersed in the light transmitting layer.
In this signal processing device, incident signal light is diffused by the light scatterers dispersed in the light transmitting layer, and the signal can be received and transmitted between arbitrary circuit boards optically connected with the light transmitting layer.
JP-A No. 2000-241655 discloses an optical coupler having a configuration where a transmission diffusing layer is formed on one end of an incident portion of the light transmitting layer and a plurality of optical fibers are brought into contact with the transmission diffusing layer.
A mode where a diffusing layer is used so as to diffuse incident light includes two typical modes. They are a mode in which a transmission diffusing layer is provided on an incident side of a light guide body, and a mode in which a reflection diffusing layer is provided on the other end of the light guide body. In the mode using the reflection diffusing layer, because the incident portion for the signal light is normally made of a flat light transmitting material, one ends of the optical fibers can be coupled with the light guide body by adhesive or the like. For example, when it is demanded that a length of the light guide body is shortened by performing a diffusing function just after incidence, an area for causing diffusion is required on the incident side for the signal light.
In the light signal transmitting device in which the transmission diffusing layer is provided on the incident portion disclosed in JP-A No. 2000-241655, because the incident light is refracted and diffused by fine irregularities formed on a surface of the transmission diffusing layer, it is necessary to lay an air layer in gaps between the optical fibers and the irregularities on the diffusing layer. That is to say, in principle, an adhesive material having refractive index smaller than that of the optical fibers or the diffusing layer can be provided to the gaps of the irregularities, but actually the adhesive material, which has refractive index small enough to an extent of maintaining diffusing properties, is expensive. Because option is limited, the air layer is laid and the optical fibers are physically fixed by using a holding member or the like. In this bumping structure, however, when a resin material such as adhesive for holding the optical fibers intrudes into the air layer portion corresponding to the gaps at the time of production, the diffusing function is deteriorated outstandingly.
In the light guide body in which diffusing particles are dispersed in the entire light guide body disclosed in JP-A No. 10-123350, particles representing the light diffusing function are present just after incidence. A light signal is emitted from one ends of the optical fibers, and the incident sides of the optical fibers are connected with the light guide body. As such a connecting method, however, an advantageous method is not disclosed.